The present invention relates to a measurement method and an electron microscope.
A scanning transmission electron microscope (STEM) is an electron microscope configured to perform scan on a sample with a converged electron beam, and map, in synchronization with the scan, the strength of detection signals based on transmitted electrons or scattered electrons from the sample, to thereby obtain a scanning transmission electron microscope image (STEM image). The scanning transmission electron microscope has attracted attention in recent years as an electron microscope capable of obtaining quite high spatial resolution at an atomic level.
As an electron detector that is mounted on such a scanning transmission electron microscope, there has been known a segmented detector having a detection plane segmented into a plurality of detection regions. The segmented detector includes independent detection systems for the plurality of segmented detection regions, and each detection system only detects electrons incident on a specific detection region on the detection plane. In the scanning transmission electron microscope, the detection plane and a diffraction plane are matched with each other (are conjugate planes). This corresponds to that electrons having passed through or scattered from the sample to enter specific solid angle regions are detected. Using the segmented detector therefore has an advantage that pieces of information on the solid angle dependence of electron scattering due to the sample can be simultaneously obtained to be quantitatively evaluated (for example, see JP-A-2011-243516).
FIG. 14 is a diagram illustrating the operation of a related-art scanning transmission electron microscope 101 including a segmented detector. In FIG. 14, only the main part of the scanning transmission electron microscope 101 is illustrated.
In the scanning transmission electron microscope 101, as illustrated in FIG. 14, an electron beam EB is converged on the surface of a sample S by an illumination-lens system 102. Then, the electron beam EB having passed through the sample S is detected by a segmented detector 106 after the camera length is adjusted by an imaging lens system 104. A charge coupled device (CCD) camera 108 is placed behind the segmented detector 106.
As a method for visualizing the electromagnetic field of a sample with the use of the scanning transmission electron microscope including the segmented detector as described above, there has been known the differential phase contrast (DPC) method. This method includes measuring how much an electron beam is deflected when passing through a sample, and calculating, on the basis of the measurement result, the electromagnetic field of the sample that causes the deflection of the electron beam.
When measurement is performed by the DPC method, the direction of a detection region of the segmented detector needs to be adjusted with respect to a STEM image. When the direction of the detection region of the segmented detector in the STEM image is unknown, the direction of an electromagnetic field that acts on an electron beam having passed through a sample, thereby deflecting the electron beam cannot be identified.
FIG. 15 is a diagram illustrating an example of the relationship between the crystal orientations of the sample S and the relative orientations of detection regions D1, D2, D3, and D4 of the segmented detector 106. FIG. 16 is a diagram schematically illustrating an image I (D2-D4) based on a difference between a STEM image obtained in the detection region D2 and a STEM image obtained in the detection region D4, and an image I (D1-D3) based on a difference between a STEM image obtained in the detection region D1 and a STEM image obtained in the detection region D3.
As illustrated in FIG. 15, for example, the detection regions D1, D2, D3, and D4 are arranged so that the detection regions D2 and D4 are placed in a [110] direction, and the detection regions D1 and D3 are placed in a [−110] direction. Photography is performed under this state, and a STEM image is obtained for each of the detection regions D1, D2, D3, and D4. The image I (D2-D4) and the image I (D1-D3) illustrated in FIG. 16 are then generated. An X direction of the photographed STEM image corresponds to the [110] direction of the sample S, and a Y direction of the STEM image corresponds to the [−110] direction of the sample S.
From the image I (D2-D4), which is illustrated in FIG. 16, information on deflection in the [110] direction that is caused when the electron beam passes through the sample can be obtained, and from the image I (D1-D3), information on deflection in the [−110] direction can be obtained. On the basis of the relationship between the crystal orientations and the deflection, the distribution of the electromagnetic field in the sample can be grasped.
Here, description is given on an example of a method for measuring the direction of each of the detection regions D1, D2, D3, and D4 of the segmented detector 106 in a STEM image. FIG. 17 to FIG. 23 are diagrams illustrating an example of the measurement method.
In order to grasp the direction of each of the detection regions D1, D2, D3, and D4 of the segmented detector 106 in the STEM image, first, the imaging lens system 104 is adjusted so that the surface of a sample is conjugate to a detection plane 105 as illustrated in FIG. 17. When scan is performed under this state, an image I1 having the shape of the detection plane 105 can be obtained as illustrated in FIG. 18. In addition, the segmented detector 106 is retracted, for example, and an image 12 (see FIG. 19) of a probe being scanned by the CCD camera 108 is obtained. In this way, a scan region can be confirmed. When these images are combined, the relationship between the direction of each of the detection regions D1, D2, D3, and D4 of the detection plane 105 and the direction of the CCD camera 108 is grasped as illustrated in FIG. 20.
Next, the scanning transmission electron microscope 101 is returned to the setting for obtaining a STEM image illustrated in FIG. 14. Under this condition, as illustrated in FIG. 21, an image 14 that is an image of the shadow of the illumination system's aperture (not shown) is observed. When defocus is added so that the detection plane 105 is shifted from the diffraction plane, the shadow of the aperture is moved along with the scan. For example, when photography is performed with long exposure time by performing scan only in the X direction, as illustrated in FIG. 22, an image I5 that indicates the locus of the shadow of the aperture is obtained. From a direction A of the movement of the shadow of the aperture and the direction of each of the detection regions D1, D2, D3, and D4 of the detection plane 105 in an image 13, which is illustrated in FIG. 20, the directions of the detection regions D1, D2, D3, and D4 of the segmented detector 106 in the STEM image can be grasped as illustrated in FIG. 23. The direction A illustrated in FIG. 23 corresponds to the X direction of the STEM image.
In this way, in the above-mentioned method for measuring the direction of the detection region in the STEM image, the direction of the detection region with respect to the STEM image (scanning direction) is measured in the two stages, that is, the direction of the CCD camera with respect to the segmented detector, and the scanning direction with respect to the segmented detector. Thus, in such a measurement method, it takes long time to only obtain the images I1 to I5 necessary for measurement. The direction of the detection region of the segmented detector in the STEM image is changed also when the scanning direction is changed. Performing the above-mentioned measurement every time the scanning direction is changed puts a heavy burden on a user. Further, in such a measurement method, the CCD camera is necessary.